Seepage from waste lagoons has the potential to leach into underlying groundwater systems. Groundwater flow can transport this nutrient-rich lagoon seepage to surrounding surface waters where water quality degradation may occur. Groundwater contaminated by lagoon seepage can be removed from the shallow aquifer by pumping, but the pumped water must still be treated before discharge into surface waters. Constructed wetlands have been successfully implemented to attenuate nitrogen and phosphorus in wastewaters. A constructed wetland receiving groundwater contaminated by swine lagoon seepage was monitored for four years during this study. Wetland effluent concentrations were variable but were on average substantially lower than influent concentrations. During the four-year study, the wetland assimilated between 76% and 84% of the influent total nitrogen and between 22% and 42% of the influent total phosphorus, or 520 to 575 kg of total nitrogen per hectare per year and 83 to 158 kg of total phosphorus per hectare per year. Residence time likely played a major role in nutrient assimilation within the wetland.